The physical mechanism for generating the field-free orientation is to create a rotational wave packet, which consists of a superposition of rotational states with even and odd angular momentum quantum numbers. Experimentally, all-optical techniques in generating such a field-free orientation have become mature thanks to the non-resonant light-interactions with molecular polarizability, and hyperpolarizability [1]}, [2]}, [3]}, [4]}, [5]}. As compared with the all-optical techniques, the progress in the realization of the field-free orientation of molecules exclusively using resonant terahertz pulses was relatively slow [6]}, [7]} since it was proposed 20 years ago [8]}, [9]}, [10]}. The technical difficulty was previously attributed to using a highly asymmetric half-cycle terahertz pulse, which impulsively transfers angular momentum to the molecule due to a non-zero (time-integrated) area of the short central part [11]}, [12]}, [13]}, [14]}, [15]}, [16]}. Further theoretical investigations have shown that molecules can be oriented through resonant excitation using symmetric terahertz pulses with a zero-time-integrated area. In addition, applying an intense non-resonant ultrashort pulse to align the molecule prior to the resonant terahertz excitation, comprehensive theoretical research has recently resulted in the experimental demonstration of enhanced field-free orientation in the sudden-impact limit [17]}, [18]}, [19]}, [20]}. However, it remains challenging to obtain a robust field-free orientation without the additional use of an intense non-resonant pulse or a static electric field.
